The present invention pertains to digital switching systems and more particularly to a large capacity fault tolerant time switching stage of a digital switching system.
The technology of telephone switching centers has proceeded from mechanical to electromechanical to solid state technologies. The network portion of the switching centers has kept the pace with this change of technologies. The network of large switching systems has gone from a mechanical network to an analog electromechanical network to the present modern day solid state digital networks.
Due to the public policy of requiring telephone switching centers to operate 24 hours a day continuously without disruption of service, highly reliable switching systems are required. This requirement causes added complexity for telephone switching centers which are CPU controlled solid state digital networks.
In addition, an increasing number of customers has provided for an increasing requirement for central switching system capability. Digital switching techniques were initially introduced via small PABX switching equipment. Following this introduction, digital techniques were employed in larger PABX's and finally in central office switch equipment.
As the demand for digital switching equipment grows, large central office switching systems are required. Further, it is required that these switching systems operate efficiently in terms of power consumed and heat dissipated and with a minimum of components. As these switching systems grow, larger channel capacities are required.
One scheme of increasing the network capacity of a digital switching system is to increase the size of the time and space switching sections of a time-space switching network. However, doubling the number time stages may increase the size of the space stage by as much as a factor of two squared or 4 times. Such a configuration means greatly increasing the number of physical components of space switching equipment.
Such a solution is taught by U.S. Pat. No. 3,991,276, issued on Nov. 9, 1976, to A. Reginer et al. This system teaches a time-space-time division switching network employing a multistage space division switch. Another time-space-time switching network is taught in U.S. Pat. No. 4,005,272, issued on Jan. 25, 1977, to A. A. Collins et al. When this system establishes a communication path in one direction, it automatically establishes a path in the opposite direction in an adjacent time slot. However, such folded network systems provide a higher percentage of blocking which greatly lessens the call handling capacity of the system.
Another folded network time division switching system is taught by U.S. Pat. No. 4,064,370, issued on Dec. 20, 1977, to H. E. Coonce et al. The space division portion of the switching network is physically large resulting in delays of digital data words transmitted through the system. In addition, the problems of a folded network as mentioned in the Collins reference above are present in the Coonce system also.
In addition, a non-folded T-S-T modular network is taught by U.S. Pat. No. 3,956,593, issued to A. A. Collins et al, on May 11, 1976.
Accordingly, it is an object of the present invention to provide a large duplex time-space-time network for a digital switching system which is low in the percentage of blocking and providing for an efficient space switching stage while handling a large number of channels.
It is another important objective of the present invention to provide for a highly reliable time-space-time network configuration in order to prevent telephone traffic disruption due to a network equipment failure.
It is a further object of the present invention to provide for a low cost of telephone service provided to each customer.
It is a further object of the present invention to provide a time switching network for a large digital switching system having the capability to increase in size modularly and provide new telephone switching services to the telephone customers.